Torque-motor driven spool valves are well known in the art including such valves which operate through the utilization of a rotary torque motor having a drive member extending from the rotor thereof into contact with the spool valve to directly reciprocate the spool valve within a bore provided in the valve housing to thereby control the flow of fluid from a source thereof to the load in response to electrical signals applied to the drive motor. Typical of such direct drive servovalves is that illustrated in U.S. Pat. No. 4,793,377 issued Dec. 27, 1988, to Larry E. Haynes et al. The invention described and claimed herein is an improvement over the direct drive servovalve disclosed in U.S. Pat. No. 4,793,377 and therefore the disclosure of U.S. Pat. No. 4,793,377 is incorporated herein by this reference.
The drive motors of such devices include a rotor and stator disposed within a housing in such a manner that the rotor assembly is subjected to the high pressure fluid typically used in servo control systems with which the device is associated. In such devices, it is desirable to have the ability to properly position the rotor to accomplish null centering of the rotor assembly. Typical of prior art devices of the type described are U.S. Pat. Nos. 4,507,634 and 4,641,812. In each of these devices, the stator position is maintained in its null by internal torsion centering springs. As is well known to those skilled in the art, as long as the drive motor is not energized, the centering springs acting on the rotor shaft will retain the rotor assembly in its null position. Moreover, after the drive motor has been activated to move the rotor assembly through its limited arcuate movement in either direction from its null position, upon de-energization of the drive motor, the centering springs will once again return the rotor assembly of its null position.